Magnetic overcoats are commonly formed on substrates, such as magnetic thin-film discs employed for recording data. The thickness of the overcoat is typically between about 200-500 .ANG. and preferably about 300 .ANG.. Greater thickness of the overcoat, and thus greater distance between the thin-film magnetic storage layer and the read/write head, tends to degrade disc resolution and storage density.
The carbon overcoat functions to protect the underlying magnetic layer from damage and wear caused by repeated contact between the disc and the read-write used in accessing the disc. For this reason, the graphite overcoat is ideally formed to have a high degree of hardness or erosion-resistance.
In addition, the graphite overcoat is intended to provide lubricating surface properties, to minimize drag on the head and wear on the disc during prolonged head/disc contact. The overcoat therefore ideally provides a low-friction surface. The lubricity of a hard carbon overcoat on a disc may be enhanced by covering the overcoat with a thin liquid layer of a stable fluid material, such as a fluorocarbon. The optimum friction reduction may be achieved with a liquid layer of fluorocarbon of about 15-30 .ANG.. With a thicker liquid layer, the head may tend to "blot" the liquid layer, producing greater drag between the head and disc with a resultant reduction in the operating lifetime for the disc.
A variety of methods have been used heretofore for forming carbon overcoats on a thin-film magnetic disc (Tsai). In one method, known as RF plasma or glow discharge, an RF source is used to decompose an hydrocarbon gas, producing a carbonaceous plasma whose carbon particles are deposited on a thin-film substrate to form the carbon overcoat (e.g., Natarajan; Yolamanchi; and Kobayashi). The RF discharge method is relatively slow, and deposition rates and plasma composition are somewhat difficult to control.
Another method which has been used for producing a carbon overcoat involves carbon deposition by DC sputtering, typically DC magnetron sputtering, in which the ionized gases are directed onto the target by magnetic fields established in the sputtering device. Typically in this method, a graphite substrate is sputtered onto a thin-layer film substrate in a low-pressure argon gas until an overcoat of the desired thickness is reached.
The resulting carbon overcoat has a predominantly graphitic structure with "islands" of diamond-like crystalline clusters with dimensions on the order of about 20 .ANG.. It is, of course, the diamond-like clusters which impart the hardness properties to the overlayer. Although the overcoat formed in this manner has adequate hardness properties, it would be desirable to increase the lubricity of the layer as well, particularly the lubricity of the overcoat after initial wear. Experiments conducted in support of the present invention indicate that carbon overcoats formed by DC magnetron sputtering in a pure argon atmosphere tend to show a substantial loss of lubricity as the overcoat is worn, in turn, causing greater wear on the overcoat. As a result, mechanical stress in the system wearing away of the disc overcoat are both accelerated.
The need for increased lubricity is especially great in the inner diameter region of the disc, where the fluorocarbon liquid coating applied to the overcoat becomes depleted over time due to migration of the liquid material under centrifugal effects, and particularly, in the inner-diameter region which is dedicated to start-stop head contact, where repeated contact with the head further depletes the liquid layer.
Various modifications of DC sputtering for use in producing carbon overcoats have been proposed. One modification, for example, is to form the overcoat by DC sputtering of a carbon target in the presence of methane or a mixture of argon and methane (RD 269061; Craig). This approach has the potential for increasing the lubricity of carbon overcoat films, as has been verified by experiments conducted in support of the present invention. However, experiments conducted in support of the present invention indicate that merely adding hydrocarbon gas to the sputtering chamber increases lubricity, but also reduces hardness, i.e., resistance to wear.
Heretofore, the development of methods for producing durable, high-lubricity overcoats for thin-film media has been hampered by slow and/or inaccurate disc testing procedures. Disc hardness is measured, according to the prior art, by a static or dynamic (moving disc) scratch test in which the depth of scratch or indentation (at a given scratch pressure applied to the disc), provides a measure of hardness. This test is difficult to quantitate, and does not necessarily correlate well with the expected start/stop lifetime of a disc.
The lubricity properties of carbon-overcoat discs are generally measured in terms of static or dynamic (rotating disc) coefficients of friction. This is done by a standard drag test in which the drag produced by contact of a read/write head with a disc is determined. One important property of a disc which is required for good long-term disc and drive performance is that the disc retain a relatively low coefficient of friction after many start/stop cycles or contacts with a read/write head. For example, a drive manufacturer may require that the disc have an initial coefficient of static friction no greater than 0.3, and a coefficient of static friction of no greater than 0.6 after 20,000 start/stop cycles. This disc specification indicates that the disc can tolerate at least 20,000 start/stop cycles without showing high friction characteristics which would interfere with read/write operations.
In fact, the above start/stop lifetime test, in which the coefficient of friction is measured before and after a large number of start/stop cycles, has heretofore been the only reliable measure of start/stop lifetime of a disc.
An obvious limitation of this test is the length of time required, as well as the wear on the testing machinery. More importantly, the results of the test do not indicate how changes in hardness and/or lubricity, as measured by prior art methods, would lead to greater durability, or more generally, how the hardness and lubricity properties of an overcoat can be varied to produce desired durability and low-friction properties in a disc.